I'm not trying to hurt him, but he had to go with the movement. I asked him his level and told him mine. To which of course he said, so what...

Maybe you were being too careful? Do you think an effective iriminage would have broken him? And on the other hand, being clotheslined isn't pleasant. So it would have been reasonable for him to be a bit upset.

At nikyu I'd really expect that one can handle a beginner even if they are taking awkward ukemi. And if things go wrong have the sense to use it as an opportunity to improve ones technique.

So look back at what happened. Maybe you didn't have his balance from the start. Then that is a thing to work on. Or he tried to recover his balance in an unexpected direction. In that case you might have to do another technique.

A good way to handle this might have been to say: "I'm sorry, lets look at what happens there" or something along those lines. Then it becomes a cooperation of you two trying to figure out what is going on in the technique, instead of an argument about who is right and who's rank is higher.